The ring laser gyro is a significant departure from prior art angular rate sensor devices. Conventional angular rate sensors employ a spinning mass to provide a reference direction. These sensors comprising spinning masses have inherent problems among which are high drift rates, caused by friction, and unwanted torques. The ring laser gyro for the most part eliminates the undesirable characteristics of the prior art sensors. Its operation is based entirely upon optical and electronic phenomena wherein angular motion is measured by the massless light waves circulating in a closed path.
The prior art (e.g., U.S. Pat. Nos. 3,373,650 and 3,467,472 to Joseph E. Kilpatrick) teaches a triangularly shaped ring laser resonant cavity defined by three corner mirrors. The triangular shape is preferred because it uses a minimum number of mirrors. A gas laser fills the laser cavity. The gas laser filling the cavity comprises Helium and Neon gas operating at one of three wavelengths, either 3.39 or 1.15 micrometers in the infrared spectral band or 0.6328 micrometers in the visible wavelength region. Through a proper choice of the ratio of the two neon isotopes Ne.sup.20 and Ne.sup.22 in the gas mixture, two monochromatic beams are created. The two laser beams propagate in clockwise and counterclockwise directions around the triangular cavity following the same closed path. With no rotation about the input axis, the cavity lengths for the two beams are equal and the two optical frequencies are the same. Rotation in either direction causes an apparent increase in cavity length for the beam travelling in the direction of rotation and a decrease for the other beam. Since the closed optical path is a resonant cavity providing sustained oscillation, the wavelength of each beam must also increase or decrease accordingly. Rotation of the ring in either direction thus causes a frequency split and the two frequencies are unequal by an amount proportional to the rotation rate. At the output mirror, the clockwise and counterclockwise beams are extracted and combined by an output prism to produce an interference pattern which is detected by a photodetector. The photodetector senses the beat frequency caused by heterodyning of the two signal frequencies. The beat frequency output is the measure of rotation rate.
All ring laser gyroscopes are sensitive to temperature gradients across their line of symmetry. Such gradients affect the Langmuir flow. The Langmuir flow, caused by cataphoretic pumping between anode-cathode, is usually well-balanced by careful machining of the capillary hole that contains the glow discharge and by the utilization of two symmetrically placed glow discharges as well as by maintaining a constant current discharge in the two glow discharges by means of two active current regulators.
The ring laser gyroscopes of the prior art are extremely sensitive to temperature changes present in the environment or temperature changes caused by warmup. These temperature changes in prior art ring laser gyroscopes cause gradients across their plane of symmetry because the gyroscope block, as taught by the prior art, was unsymmetrical. As a result, output pulses appear although there has been no rotation about the input axis. Making the block unsymmetrical was a method necessitated by the prior art to prevent lock-in by mechanical dither. Lock-in occurs at low input rotation rates, as the input rate falls below a certain critical or threshold value. In the lock-in region, a nonlinear relationship exists between the input and the output. Beyond the lock-in region, there is a substantial linear relationship between the input and output.